API: Check that form submission HTTP method matches form

- Add test to Form#httpSubmission()
- Send HTTP 405 response & Allow header on mismatch (per RFC 2616 10.4.6)
- Because HTTP methods are upper case, change the representation of methods to upper case
- Retain lower case for Form#FormMethod(), because that is intended for X/HTML
- Update FormTest to accommodate the case changes
- Update documentation
Fixes #1672